Recently, a so-called RFID (radio frequency identification) system using a contactless IC card, IC tag or the like has been introduced into the fields of art such as an automatic ticket checker used in the railway station, security system for checking people going to enter or exit from a building, electronic money system, etc. As schematically illustrated in FIG. 1, the RFID system includes a contactless IC card 100 and a reader/writer 101 to write and read data to and from the IC card 100. The RFID system adopts the theory of electromagnetic induction. An electromagnetic field radiated from a loop antenna 102 provided at the reader/writer 101 is coupled by the electromagnetic induction to a loop antenna 103 provided at the IC card 100 to provide communications between the IC card 100 and reader/writer 101.
In the above RFID system, the IC card has not to be inserted into the reader/writer to put metallic contacts into contact with each other as in the conventional contactless IC card systems. Therefore, data can be written to, and read from, the IC card easily and quickly. Also, in the RFID system, the electromagnetic field radiated from the reader/writer 101 provides a necessary power to the IC card 100 and thus any power source such as a battery or cell has not to be provided in the IC card. Therefore, the IC card used in the RFID system can be excellent in maintainability, lower in price and higher in reliability.
In the above RFID system, the loop antenna 102 provided at the reader/writer 101 should be able to radiate an electromagnetic field having a certain degree of magnetic strength in order to assure a satisfactory range of communications between the IC card 100 and reader/writer 101.
Generally, the loop antenna 102 for the reader/writer 101 includes a loop coil 200 formed from a plane winding of a conductor as shown in FIG. 2. The loop coil 200 is formed symmetric for winding sections thereof opposite to each other across the center of the loop coil 200 to be equal in interval and width to each other.
In the above symmetric loop antenna 102 for the reader/writer 101, the magnetic field is distributed symmetrically with respect to a Z-directional section through the center of the loop antenna 102, perpendicular to the long sides of the rectangular antenna and also to an X-directional section through that center, perpendicular to the short sides, as shown in FIG. 3.
FIG. 4 shows the dependence upon the card position of the strength of a current induced by the loop antenna 102 to the IC card 100. As shown, two communication areas S1′ and S2′ are formed in positions, respectively, opposite to each other across the center of the loop coil 200. More particularly, the-communication area S1′ assures an ideal magnetic coupling. Namely, magnetic fields developed at four sides of the loop antenna 102 at the reader/writer 101 are inductively coupled to those developed at four sides of the loop antenna 103 at the IC card 100, opposite to the four sides, respectively, of the loop antenna 102. Outside the communication area S1′, there is an area where magnetic fields crossing the loop antenna 103 at the reader/writer 101 cancel each other in a central area where the magnetic fields developed around the loop antenna 102 at the reader/writer 101 are inverted in direction. In this outer area, the induced current will have a lower level than the necessary level for the communications. Outside the above outer area, there is the communication area S2′ where only one of the four sides of the loop antenna 102 at the reader/writer 101 is coupled to one of the four sides of the loop antenna 103 at the IC card 100. Therefore, the communication area S2′ is narrower than the communication area S1′, and the induced current in this communication area S2′ is smaller than that in the communication area S1′.
Note that in FIG. 4, the origin “0” of the horizontal axis indicates the center of the loop antenna 102 at the reader/writer 101 and the positive-going direction indicates a direction from the center (origin “0”) toward outside of the IC card 100. The vertical axis indicates the strength of the current electromagnetically induced in the loop antenna 103 at the IC card 100 under the action of the magnetic field in the loop antenna 102 at the reader/writer 101. Communications are possible in an area where the strength of the induced current has a value larger than a value indicated with a dashed line s′ in FIG. 4.
Note here that when the communication area S1′ is continuously wider as far as possible outwardly of a point, namely, the origin “0”, where the center of the loop antenna 103 at the IC card 100 coincides with that of the loop antenna 102 at the reader/writer 101, the RFID system will be easier to use.
That is to say, in a direction the origin “0” toward outside of the above conventional loop coil 200, the communication area S1′ is followed by a non-communication area once, and then by the communication area S2′. It is desirable from the practical point of view that no non-communication area should exist between the communication areas S1′ and S2′ or only the communication area S1′ should spread.